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Taxonomy is the realm of science that attempts to group similar organisms together, which is not always an easy task. In this lesson, we will examine the protozoa, a diverse group of organisms that didn’t really fit anywhere else.

Early Classification Problems

Carl Linnaeus is thought of as the father of modern taxonomy. It was Linnaeus who came up with the two-word descriptions of organisms: the genus and species names that all scientists are familiar with. At first, organisms were placed in groups based solely on what could be observed: appearance, behavior, and ecological niche.

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But, what happens when you observe thousands of organisms, create hundreds of distinct groups, and end up with a bunch of single-celled oddballs that don’t really fit anywhere else?This was the problem facing noted plant ecologist Robert Whittaker in the 1950s when he proposed his plan for a five-kingdom taxonomic system. Without the ability to sequence and compare the genomes of these oddballs, all he could do was guess where to put them. In a decision borne more out of convenience than accuracy, he stuck them all in the Kingdom Protista and moved on. What resulted was a group of distantly related organisms forever lumped together and collectively called ‘protozoa.


The group of organisms known as ‘protozoa’ are defined by a few of their shared characteristics. Protozoa are non-phototrophic, unicellular, eukaryotic microorganisms with no cell walls. This diverse group of over 65,000 species generally share these basic attributes. Looking deeper, this group can be extremely complex and variable. In fact, the protozoa are often described as the pinnacle of unicellular complexity.

Unlike the relatively simple bacteria, protozoa can have many different intracellular organelles performing specific tasks. Some species of protozoa have structures that are analogous to mouths, GI tracts, and anuses. This probably goes against everything you’ve been taught about microbes being simply bags of proteins and enzymes.Many protozoa cause diseases in animals and humans. Some, like Plasmodium, which causes malaria, can be devastating to people worldwide.

Others, like Trichomonas, cause sexually transmitted diseases that are relatively benign and 100% curable. The vast majority of the species, though, are completely harmless. But, as is usually the case in microbiology, it’s the dangerous ones that get the most attention.The protozoa can have very diverse lifecycles with multiple morphological stages, depending on species.

Most protozoa have a cyst stage, which is dormant and highly resistant to environmental stress. In the disease-causing species, these cysts are often the mode of infection, frequently acquired by fecal-oral contamination. The trophozoite stage is the active, reproductive, and feeding stage. The image below shows a purple trophozoite emerging from an oblong brownish-red cyst.

A trophozoite emerging from a cyst
prey.Another interesting characteristic of the ciliates is the presence of two nuclei.

A macronucleus houses most of the genome and is responsible for directing the basic cell processes. There is also a micronucleus that is only involved in sexual reproduction and genetic inheritance. Ciliates without a micronucleus, lost either naturally or removed by scientists, are unable to reproduce sexually but can still replicate asexually.The most common genus within the ciliates is the genus Paramecium. This ciliate is found in nearly any environmental water sample, making it a convenient organism to study in introductory biology classes. Paramecium is also harmless, ensuring safe examination by budding young scientists.


The last group is the Apicomplexa, more commonly called Sporozoans.

This group is quite different than the other three. This is the only group that have mature forms that are non-motile. Sporozoans lack flagella and cilia and are unable to make pseudopods.

All sporozoans are obligate parasites, meaning they must live in association with a host organism. This group can often have complex lifecycles that require multiple host species.Many sporozoans cause disease.

Of all the diseases caused by protozoa, the deadliest is malaria, caused by the sporozoan Plasmodium. When a female Anopheles mosquito bites a human, it is able to transmit Plasmodium into the bloodstream. Malaria develops when the Plasmodium invades the host’s red blood cells, replicates itself, and bursts all the infected red blood cells at once. Malaria still kills over 600,000 people every year, mostly children under the age of 5 that live in Africa.Toxoplasma and Cryptosporidium are two more examples of disease-causing sporozoans that infect humans.

Lesson Summary

It’s time to review.Protozoa are a diverse group of organisms that are non-phototrophic, unicellular, eukaryotic microorganisms with no cell walls. In general, protozoa have different stages in their lifecycles. Trophozoite is the active, reproductive, and feeding stage. In the cyst stage, the organism is dormant and highly resistant to environmental stress. Some lifecycles require specific species to serve as hosts.

Others are more general, able to infect a large number of different hosts.Still others require two different organisms to complete their lifecycle.Protozoa are divided into four main groups based on how the organism moves. The Flagellates move by waving long, whip-like flagella.

Trypanosoma and Giardia are common flagellates. The Amoebas move by pseudopod action. Entamoeba histolytica is a common amoeboid pathogen.The Ciliates are a large group that move by waving cilia. This predatory group is generally harmless, targeting bacteria and other protozoa, with very few causing disease in humans. Paramecium is the classic ciliate.

The Sporozoans are non-motile. They are obligate parasites, often with complex lifecycles. Malaria, the deadliest protozoal disease, is caused by the sporozoan Plasmodium.

Learning Outcome

At the conclusion of this lesson, you’ll be able to:

  • Describe the general characteristics of protozoa
  • Explain the lifecycles of protozoa
  • List the four main groups of protozoa and state how the groups are classfified
  • Summarize the specific characteristics and provide examples of each group: Flagellates, Amoebas, Ciliates and Sporozoans

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